It shouldn't have surprised him that Lestrade would drop by 221B to pick up the case. Lestrade wasn't dumb enough to think that Sherlock wouldn't find it, so Sherlock should have expected to find the one detective of Scotland Yard in his living room at one point.
However.
He hadn't expected to find that that certain detective had broken into his flat.
"You can't just break into my flat!"
"And you can't withhold evidence! And I didn't break into your flat."
"Well, what do you call this, then?"
"It's a drugs bust."
Oh, that was clever. That was clever, because only Lestrade knew that Sherlock having drugs in the flat was totally possible. Traitor. Backstabbing traitor.
"Seriously? This guy, a junkie? Have you met him?"
Sherlock's attention snapped back to John without a second's pause. Stupid John. John knew nothing about Sherlock. He shouldn't assume, so he should just shut his mouth.
"John."
"I'm pretty sure you could search this flat all day and you wouldn't find anything that you could call recreational."
"John, you probably want to shut up now."
"Yeah, but come on..."
Sherlock instantly noticed the look that overtook John's eyes. He instantly knew what was coming next. He knew instantly that he wouldn't own up to it.
"... No."
"What?"
"You?"
"Shut up."
It wasn't any of his business, anyway.